I would try something a little more personal.
We are pleased to now be able to offer our customers the secure online payment gateway of Paymate to safely process their transactions.
Paymate allows us to process your payment much quicker, in fact in around 24 hours, and that means your item is shipped quicker with absolutely no unwanted, and frustrating shipping delays due to slow payment processing.
Try sweetening the deal, 5% off Paymate Transactions, add 2.9% to PayPal transactions to cover you cost, delay you PayPal shipping, nothing shipped within 3 working days.....lots of different tools you can choose to press the right button in a customer.
The trick is, they always have them think that it's their own idea. If they get a smell that it will assist you, they'll stay clear!
OK, just something off the top of my head, but the idea is to make them feel like you have added the service for their benefit, and that the service is just as safe as what they are currently using.
At the end of the day a customer wants to use the service that serves them best, you need to make that link for them, otherwise they will just stay with what has been servicing them best in the past.